I just landed a patch to support domino tiles (aka Wang Tiles). The idea follows from the game of dominoes: each tile edge has a label and labels must match between adjacent tiles. Why, as a tile artist, should you care? It frees you from the monotony of forcing every edge to match. Currently we support the tile set described in this paper, but this is easily expanded. This set provides two colors horizontally and two vertically.

If this description isn't much help, try this image:Notice how colors are preserved across tile boundaries!

Here's an actual tileset in crypt:

So, how can you create tiles? Here is a gallery of templates. For each template, you should make at least one corresponding tile.